The present invention relates to methods and apparatus for pasteurizing or sterilizing items. More particularly, the invention relates to improved cooking systems, preferably microwave cooking systems, having a plurality of linearly aligned segments for processing food products.
On the practical side, food processing techniques include a cycle of selection and precooking of foods followed by a preservation cycle, which typically includes the use of refrigerators or freezers or retort canning and, in more recent techniques, rapid heating vessels. In food preservation, food can be pasteurized and/or sterilized to reduce the occurrence of food born diseases caused by harmful microorganisms. Pasteurization involves heating food to a temperature, typically between 80° C. and 100° C., sufficient to kill certain pathogenic bacteria and microorganisms. In sterilization, food products are heated to a higher temperature, typically between 100° C. to 140° C., to ensure elimination of more resistant microorganisms and spores. Under practical conditions, to carry out sterilization, it is necessary to heat the product to a temperature above 121° C. for a time of between 5 and 12 minutes. Preferably, the product is subsequently subjected to the most rapid possible cooling to a temperature equal to or less than 35° C. Pasteurization and sterilization, collectively referred to herein simply as “food processing”, allows perishable food products to be stored in refrigerated or room temperature conditions for an extended period of time.
More traditional methods for pasteurizing and sterilizing food products involves use of convectional heating processes in which thermal energy is transferred to the food product in a high or medium temperature environment utilizing hot air, hot water, or hot vapor within an oven-type construction. In some cases, where preservation on an industrial scale is required, a post pasteurization or sterilization phase is required which may not be limited to attenuation of microbial, pathogenic and enzymatic activity, but has the purpose of destroying all microorganisms present in the product, and also in the actual container/package. This occurs, because the degree of resistance to heat of microorganisms is related to external and environmental factors, like the initial microbial concentration of the medium, the characteristics of the medium itself and the time and temperature parameters, as well as intrinsic factors related to heat sensitivity of germs and development stage of the cells, in which specific variations often occur. For example, under identical environmental conditions, it is observed that fungi and yeast are more resistant than e. coli bacteria and, within the latter, the rod forms are more resistant that the coccal forms.
In order to efficiently effect pasteurization and/or sterilization, microwave heating has been employed for pasteurization and sterilization. Advantageously, microwave heating can provide for pasteurization and/or sterilization in a much shorter time period than by employing conventional heating processes. By decreasing the food heating time, food generally tastes better and nutrient retention is improved. In addition, microwave systems typically are more energy efficient than conventional heating systems.
Great Britain Patent No. 1103597 (Newton et al.) describes a microwave heating system for controlling microorganisms in prepared foods and beverages. It prescribes for exposure of the already prepared foods to electromagnetic waves with a frequency of 20-40 MHz at an intensity of 500-3000 volts for a sufficient period of time to attenuate the microorganisms present in the manufactured product. The use of microwave energy to sterilize materials is known in even greater detail. For example, WO0102023 (Korchagin) proposes a generator that has the capacity to implement the intensity of the electromagnetic field at a level to ensure destruction of microorganisms.
Complex apparatuses, specifically continuous treatment tunnels for sanitization of packaged products, have been known since at least 1973. For example, U.S. Pat. No. 3,747,296 (Zausner) proposes an apparatus with linear development, in which filled containers are introduced and subsequently closed. The containers are passed through the tunnel, which is subdivided into different treatment zones at temperatures between 90° C. and 150° C. Means of irradiation are also provided, which have the purpose of sterilizing the cover only. Similarly, U.S. Pat. Nos. 5,066,503; 5,074,200; 5,919,506 and 6,039,991 issued to Ruozi describe conveyor driven microwave processing plants for pasteurizing, cooking and sterilizing food products. The systems include a plurality of chambers wherein the temperature and pressure are controllably elevated and decreased as the food products travel from chamber to chamber.
U.S. Pat. No. 3,889,009 (Lipoma) describes a conditioning tunnel for foods previously prepared in bowls and sealed under pressure. At the entry and exit of this tunnel, corresponding to the crossing point of the manufactured vessels, pressure closure doors are provided. Once the sealed vessels have entered the interior of the tunnel, each vessel undergoes a sterilization treatment, passing beneath a source of electromagnetic waves. Each vessel is then transferred downline, always by means of a common belt or chain conveyor, to pass through a cooling unit. A device to generate pressure during the sterilization phase operates within the apparatus to avoid a situation in which the products, because of the process, burst because of the dilation effect, or from seal failure. This phenomenon most frequently entails escape of liquid from individual containers, producing not insignificant drawbacks within the apparatus, like accumulation of dirt and the subsequent need to carry out frequent maintenance.
Other microwave tunnel apparatuses are also known. For example, Italian companies Modo Group International from Brescia Italy and Micromac from Reggio Emilia developed automatic and computerized food processing tunnels which provided for receiving the products, in this case prepared dishes in a heat-sealed vessel, and for carrying out the fundamental phases of sterilization treatment. The tunnels include elongate cylindrical constructions have diametrically round cross sections, within which, corresponding to the different stages, the following process phases were conducted: 1) preheating; 2) sterilizing using devices that generate microwaves; 3) holding or stabilization of the product at the sterilization temperature for a specified time (microwave sources, which are positioned along the lower side of the conditioning tunnel beneath or corresponding to the plane of advance of the prepared foods, are typically provided to execute at least these last two phases); and 4) cooling before unloading. At the end of the process, a finished product emerges, sterilized and ready to be packaged and stored in warehouses.
More recently, U.S. Pat. No. 7,119,313, assigned to Washington State University Research Foundation, describes a tunnel for pasteurizing and/or sterilizing food. The tunnel includes a plurality of chambers in which microwave sources are positioned on each side of a microwave cavity in which food products are heated. This reference describes pressurizing the microwave cavity and filling it with a liquid in an effort to balance the pressure generated in the food packaging to therefore prevent the packaging from bursting or otherwise opening.
Unfortunately, the aforementioned food processing systems suffer from numerous disadvantages. Foremost, though several of the prior art references describe pressurized food processing chambers, none of the references describe an efficient manner for introducing products into a microwave tunnel without a significant loss of internal pressure to the external environment as packages are introduced. Presumably in practice, the prior art systems require that the pressurizeable chambers be opened for the introduction of food products. Thereafter, the chambers are closed and pressurized to a desired pressure while subsequently or simultaneously activating a microwave heating source for heating the products. Presumably, the food products are expelled from the food processing system in similar manner requiring that the heating chambers be depressurized, one or more doors opened and the products transported from inside the heating chamber to exterior to the heating chamber. The negative implications from this time consuming and complicated intermittent process has made commercialization of pressurized microwave pasteurization and sterilization impractical.
Thus, there is plainly a need for a food processing system which will efficiently and economically provide for the introduction and transportation of food products through a pressurized microwave cooking process.
Advantageously, the food conditioning system would not require the time consuming and impractical process of continuously pressurizing and depressurizing various heating cavities for the introduction and expulsion of food products and their packages.
Even more advantageously, a preferred food conditioning system would provide for efficient holding and cooling chambers for maintaining and cooling products at desired temperatures while permitting the products to continuously move, without starting and stopping, to increase efficiency.